U^l, 


University  Library 
University  of  California  •  Berkeley 


Geodetic  Instruments  of  Precision 

— AT  THE — 

Paris     Exposition    and     in     European 
Workshops. 


DAVIDSON. 


'I  III     |;/-'i  '  fH  'I    I   H;UA 


J)3 


[Read  before  the  National  Academy  of  Sciences,  November,  1878,  at  New  York,   by  Prof. 
George  Davidson,  A.  M.,  D.  Ph.,  United  States  Coast  Surrey.] 


Geodetic    Instruments    of    Precision    at    the    Paris    Exposition 
and   in   European   Workshops. 


It  is  a  fact  familiar  to  those  who  have  occasion  to  make  use  of  instrumental 
contrivances,  that  novel  methods  are  constantly  being  devised  to  meet  new 
requirements  or  to  lessen  existing  defects.  As  the  delicacy  and  importance  of 
any  given  work  is  increased,  the  observer  soon  discovers  sources  of  error  that 
had  not  been  suspected,  and  finds  errors  that  were  veiled  by  the  disposition 
of  the  relative  parts  of  the  instruments.  Instruments  of  the  higher  class  are 
too  frequently  at  fault,  but  the  ingenious  observer  is  soon  led  to  investigate  the 
sources  of  trouble,  and  either  to  obviate  them  or  to  seek  for  the  best  instru- 
ments suited  to  his  particular  and  peculiar  duties.  In  the  broadest  view  ot 
the  case  we  must  not  be  confined  to  individuals  or  to  nationalities,  but  aim  only 
to  obtain  the  highest  mechanical  design  and  workmanship.  In  the  progress  of 
the  geodetic  work  of  the  United  States,  stretching  from  the  Pacific  to  the  At- 
lantic, and  also  northward  and  southward  along  the  Pacific  coast,  there  was 
furnished  to  me,  among  other  instruments,  one  theodolite,  of  large  size,  which 
was  subjected  to  the  most  rigid  scrutiny  to  determine  the  flexure  of  the  pillar 
plate,  of  the  microscope  arms,  and  of  the  telescope;  also,  the  character  of  the 
graduation  of  the  20-inch  circle,  and  various  other  points  involving  accuracy. 
Under  ordinary  circumstances  the  instrument  would  have  passed  muster  for 
lines  of  moderate  extent,  but  when  lines  of  160  miles  in  length  were  involved, 
it  was  essential  to  the  integrity  of  the  undertaking,  that  errors  of  even  a  second 
of  arc  should  be  avoided.  At  the  distance  of  160  miles  one  second  of  arc 
subtends  four  feet:  a  skilled  observer  can  measure  a  much  less  quantity  than  one 
second,  and  certainly  the  instrument  should  afford  the  means  of  his  measuring 
what  he  can  see. 

Fully  alive  to  the  importance  of  the  subject,  the  Superintendent  of  the  U. 
S.  Coast  Survey  had  determined  that  there  should  be  made  a  careful  examina- 
tion and  study  ot  the  instruments  of  precision,  exhibited  at  the  Paris  exhibition 
for  geodetic  and  astronomical  work,  topography  and  hydrography,  for  the  tele- 
graphic determination  of  longitude,  etc.  And,  moreover,  that  the  examination 
should  be  extended  to  the  first-class  workshops  of  Europe,  where  instruments 
of  this  character  are  manufactured. 

It  was  desired  to  ascertain,  if  practical,  whether  the  productions  of  European 
manufacturers  were  superior  to  our  own;  wherein  lay  any  superiority,  if  found 
to  exist;  and  in  what  consisted  the  peculiar  merit  of  the  work  of  any  particular 
manufacturer. 

As  a  mechanician  seeking  merely  the  trade  secrets  of  the  makers,  I  could 
not  have  expected  to  enter  into  any  sanctum  sanctorum;  but  upon  a  candid  ex- 
planation of  my  purpose  and  instructions,  I  found  nearly  every  workshop 


2 

opened  to  me,  and  Ml  and  free  explanations  made  to  my  inquiries.  It  would 
therefore  be  unjust  to  these  manufacturers  to  severely  criticise  in  public  the 
character  or  even  the  minor  defects  of  their  instruments,  or  to  make  known  the 
methods  of  their  processes.  These  I  received  for  the  benefit  of  the  Govern- 
ment, and  whenever  I  mention  any  names  here,  it  will  be  to  commend  their 
work;  although  many  are  officially  commended  that  are  not  now  referred  to. 

In  previous  experiments  upon  the  larger  and  finer  instruments,  I  have  dis- 
covered errors  of  graduation  and  flexure  of  parts  much  greater  than  I  had 
any  reason  to  suspect;  and  I  believe  much  greater  and  more  serious  than  the 
makers  had  thought  possible.  This  latter  is  the  more  readily  understood 
when  we  reflect  how  few  mechanicians  are  actually  observers;  and  that  it 
really  requires  long  practice  for  the  most  skilled  observer  combined  with 
fair  mechanical  instincts  to  discover  and  measure  the  minute  and  conflicting  er- 
rors which  are  resultants  of  diflferent  infinitesimal,  and  perhaps  unsuspected 
causes.  As  a  rule,  the  observer  makes  his  measures  upon  objects  subject  to  many 
extraneous  and  disturbing  causes,  and  whenever  unsatisfactory  results  are  ob- 
tained, he  is  very  apt  to  attribute  them  to  the  unfavorable  conditions  of  the 
atmosphere  or  to  his  own  condition  and  temperament  at  the  time;  generally 
overlooking  the  fact  that  the  instrument  maker  was  quite  as  liable  as  himself  lo 
errors  of  judgment  in  the  proportions  and  workmanship  of  the  instrument  used. 

Setting  aside  for  the  present  the  peculiar  adaptness  and  fitness  of  the 
observer  for  his  business,  we  are  necessarily  interested  in  the  requirements  of 
geodetic  operations  and  especially  in  the  duties  of  the  instruments  by  which 
these  are  satisfied.  In  all  geodetic  work,  portability,  accuracy  and  mainte- 
nance of  instrumental  adjustment  are  essential  to  rapidity  of  progress,to  economy 
of  expenditure  in  money  and  in  personnel,  and  to  precision  in  the  results. 
Experience  and  theory  teach  us  that  in  any  given  instrument,  such  as  a  theodo- 
lite for  geodetic  purposes,  we  need  simplicity  of  design;  fewness  of  pieces; 
harmony  in  the  proportion  of  parts;  accuracy  of  workmanship;  superior  gradu- 
ation with  adequate  microscope  micrometers;  micrometer  screws  free  from 
mechanical  defects;  commensurate  optical  conditions  of  penetration  and  power; 
sensitive  and  trustworthy  levels;  and  the  highest  precision  in  all  the  bearings  of 
the  moving  parts.  Moreover,  the  general  disposition  of  the  parts  should  be  such 
as  to  oflfer  the  greatest  facilities  to  the  observer,  in  order  that  he  may  make  the 
necessary  observations  without  fatigue  or  nervous  strain. 

Guided  by  these  general  considerations,  I  examined  the  fine  collection  of 
geodetic  instruments  exhibited  by  the  Minister  of  War,  many  of  the  exhibits 
of  private  exposants,  and  some  of  the  workshops  of  the  manufacturers  in  Paris. 
After  this  I  visited  the  principal  manufactories  of  Geneva,  Neuchatel,  Munich, 
Vienna,  Dresden,  Berlin,  Hamburg,  Cassel,  London  and  York.  At  the  exposi- 
tion I  did  not  have  the  fullest  facilities  afforded  me,  and  not  only  was  I  unable 
to  get  into  one  of  the  principal  cases,  but  I  was  positively  forbidden  to  con- 
tinue my  drawings;  whilst  one  well  known  maker  would  not  only  not  permit 
me  to  make  any  tests  of  the  graduation  of  his  theodolite,  but  would  not  allow 
me  see  his  graduating  engine.  Outside  of  Paris  I  was  permitted  to  see  every 
graduating  engine  of  the  noted  manufacturers,  and  afforded  facilities  for  the  ex- 
aminations of  their  productions.  But  on  account  of  the  commercial  depression 
which  was  overshadowing  all  business  in  Europe,  as  well  as  the  United  States, 
the  number  of  the  larger  instruments  on  hand  was  very  few.  In  some  work- 
shops I  found  that  not  over  20  per  cent,  of  the  usual  number  of  workmen  were 
employed. 

Besides  the  mechanical  construction  of  the  instruments,  I  was  particularly 
anxious  to  study  the  capacities  of  the  different  graduating  engines,  in  order 
to  judge,  in  a  measure,  of  the  probable  value  of  the  results.  The  graduating 
engine  is  simply  a  mechanical  tool  with  which  we  should  expect  to  divide  a  circle 
of  say  20  inches  in  diameter  in  360  parts,  with  no  greater  an  error  than  the 
1-50,000  part  of  an  inch  or  4-10  of  a  second  of  arc  in  any  one  degree.  The 
probable  error  of  an  experienced  observer  in  reading  the  five  minute  gradu- 
ations of  such  an  instrument  is  about  1-10  of  a  second  of  arc,  or  the  1-200,000 
of  an  inch.    It  is  usually  assumed  that  the  graduation  errors  are  not  over  one 


3 

second  of  arc,  and  this  is  what  the  majority  of  instrument  makers  suppose 
or  assert  to  be  their  average  error  in  each  degree  graduation;  but  I  am  per- 
fectly satisfied  that  even  this  limit  of  accuracy  is  rarely  if  ever  reached.  As  a 
matter  of  fact  I  failed  to  learn  from  any  manufacturer  that  he  had  ever  tested 
a  theodolite  after  graduation  by  measuring  every  five  minute  space  on  the  circle, 
or  even  every  degree;  whereas  in  my  previous  testings  of  a  20-inch  theodolite 
I  had  detected  differences  of  15  seconds  between  whole  degrees,  and  errors  of 
5,  6  and  7  seconds  in  contiguous  five  minute  spaces. 

You  can  readily  understand  the  almost  multitudinous  sources  of  error 
against  perfecting  a  graduating  engine;  and  the  most  skilled  mechanicians  find 
that  it  requires  persistent  labor  and  experiment  for  two,  three  or  more  years 
to  approximately  effect  their  purpose.  Yet  until  we  get  a  reliable  graduation 
it  is  futile  to  seek  for  final  accuracy  in  our  measures;  nevertheless  all  other 
sources  of  error  should  be  reduced  to  minima,  and  the  perfection  of  graduation 
persistently  attempted.  After  a  graduating  machine  has  been  made  as  nearly 
perfect  as  the  means  and  skill  of  the  mechanician  can  effect  it,  he  carefully  meas- 
ures the  different  degrees,  tabulates  the  errors,  and  generally  constructs  an 
error  circle  whose  circumference  is  irregular  in  a  certain  proportion  to  the 
errors  determined.  This  circumference  is  so  connected  with  the  tangent  screw 
moving  the  graduate  circle  as  to  advance  or  retard  the  screw  the  exact  amount 
of  the  error  at  any  given  point.  This  is  one  way  of  effecting  the  necessary  cor- 
rection, and  i3  mentioned  merely  to  serve  as  an  illustration;  yet  in  this  method 
it  must  be  evident  that  the  retarding  of  a  screw  brings  into  operation  any 
backlash  that  may  exist  even  if  it  be  almost  practically  infinitesimal. 

Among  the  instrument  makers  I  found  the  most  positive  and  opposite 
views  of  practice;  for  instance,  one  believes  wholly  in  automatic  moving  parts 
in  the  graduating  engine,  and  makes  every  effort  to  secure  uniformity  of  tem- 
perature, etc.;  another  scouts  the  automatic  movement  and  does  everything 
by  hand,  with  or  without  much  attention  to  change  of  temperature;  ore  party 
believes  in  a  steel  cutting  tool;  another  pins  his  faith  upon  his  diamond  cutter, 
etc.  Many  of  the  observers  implicitly  receive  the  dicta  of  the  instrument 
makers,  and  but  few  make  any  exhaustive  examiaations  for  themselves.  Among 
the  tests  which  had  been  made  by  the  Chief  of  the  Prussian  Geodetic  Survey, 
between  theodolites  of  a  few  of  the  best  makers,  there  was  a  very  close  agree- 
ment in  the  probable  error  of  the  systematic  errors,  but  the  probable  error  of 
the  irregular  errors  was  only  half  as  great  for  the  Wanschaff  circles  as  for  the 
others.  Without  here  mentioning  the  rames,  I  may.  state  that  from  the  charac- 
ter of  the  instruments  examined,  I  recommended  the  Superintendent  of  the 
Coast  Survey  to  have  circles  graduated  by  five  of  the  best  makers,  and  test 
them  as  I  had  tested  the  20-mch  theodolite  referred  to.  For  where  mechanical 
si^ill  is  good  and  apparently  nearly  equal,  it  would  be  vain  to  attempt  to 
decide  by  simple  inspection  or  upon  a  maker's  repatation,  although  upon  these 
merits  alone  awards  were  made  at  the  exposition. 

After  the  instrument  maker  has  efTected  the  division  of  the  graduating 
circle  into  degrees,  the  sub-division  into  2-minute,  4-minute,  5-minute,  or  10- 
minute  spaces  is  sometimes  made  by  verniers  suitably  divided,  but  frequently 
the  coincidence  of  the  lines  of  the  vernier  and  the  circle  is  determined  by 
bringing  the  ends  of  the  lines  together,  and  judging  by  the  eye  whether  one  is 
truly  a  prolongation  of  the  other.  The  Kepsolds  certainly  appreciated  this  source 
of  error  and  had  in  a  great  measure  successfully  overcome  it.  Some  of  the 
graduating  engines  examined  had  been  made  and  in  use  from  thirty  even  to 
fifty  years;  now  considering  the  great  advance  in  the  construction  of  instru- 
ments of  precision  in  that  time,  it  is  hardly  too  much  to  ask  that  this  special 
mechanical  tool  should  be  perfected.  Of  course  there  are  more  recent  efforts 
but  not  yet  notably  perfected.  It  offers  to  our  younger  skilled  mechanicians  a 
very  delicate  problem  upon  which  to  try  their  ingenuity,  skill  and  patience; 
with  such  a  tool  superior  to  those  of  their  competitors,  they  may  be  assured  of 
a  fair  income  from  its  work  alone,  and  an  enviable  reputation. 

Of  the  larger  theodolites  which  I  had  an  opportunity  of  examining,  the 
best  was  by  Brunner.    It  combined  fewness  and  simplicity  of  parts,  and  fair 


harmony  of  proportion,  yet  I  should  certainly  doubt  the  adequacy  of  its  opti- 
cal power  for  long  lines;  it  reads  by  four  verniers  instead  of  three,  involving 
more  labor,  and  less  reading  points  on  the  circle;  whilst  the  observer  must 
be  placed  in  a  constrained  position  because  the  horizontal  plane  ofthe  ;ele- 
scope  is  only  about  two  inches  above  the  reading  microscopes.  Moreover,  the 
circle  (instead  of  the  whole  instrument)  is  changed  for  new  positions  and 
clamped  by  three  screws  in  each  position.  Should  the  surface  planes  of  con- 
taot  be  warped,  this  clamping  would  warp  the  circle  in  each  new  position  and 
thereby  introduce  error.  This  method  was  devised  about  20  years  since,  and 
soon  abandoned;  it  is  now  the  fashion. 

In  the  evident  desire  to  obtain  compactness  and  simplicity,  the  instru- 
ment is  not  adapted  to  the  observing  of  azimuths  by  means  of  a  close  cir- 
cumpolar  star,  because  the  telescope  cannot  be  ele/ated.  Instead,  the  transit 
instrument  w  isi  replace  the  theodolite  for  that  purpose,  whereby  the  possibility 
of  error  is  introduced  in  occupying  the  identical  station;  and  even  it  this  be 
granted,  the  labor  is  increased  and  extra  time  consumed. 

Fashions  prevail  among  instrument  makers  and  observers  just  as  we  find 
a  change  of  opinion  upon  the  question  of  refractors  and  reflectors.  At  one  time 
it  seemed  as  it  the  prismatic  telescope  was  to  carry  everything  before  it,  and 
I  find  many  manufacturers  and  observers  yet  strongly  in  favor  of  that  form. 
It  certainly  has  the  dec'ded  advantage  of  comfort  to  the  observer;  but  from 
personal  experience,  and  from  conferring  with  such  observers  as  Plantimour, 
it  seems  that  the  system  ncessarily  in/olves  "flexure"  or  deformation  of  the 
prism,  notwithstanding  the  njmerous  and  ingenious  efforts  made  to  secure  it 
properly  in  position.  Of  course  some  manufacturers  insist  that  their  special 
methods  of  securing  the  prisms  are  infallible;  but  the  observer  is  the  final 
jtt^ge  of  their  failures.  In  the  highest  character  of  wo  ^k  I  should  certainly  not 
use  them;  whilst  for  the  secondary  works  it  seems  hardly  necessary  to  increase 
the  number  and  Intricacy  of  the  parts. 

A  prevailing  fashion  at  present  is  to  introduce  reversing  apparatus  and 
counterpoises,  even  in  theodolites  with  circles  of  eight  inches  in  diameter. 
This  is  as  complete  a  wandering  from  simplicity  as  can  well  be  imagined,  and 
must  certainly  lead  to  grave  errors.  It  is  well  known  that  by  moving  a  tele- 
scope in  altitude  by  means  of  the  usual  slow-motion  screw,  the  tendency  is  to 
raise  the  transit  axis  pivots,  but  as  the  weight  of  the  telescope  is  too  great  to  be 
lifted  clear  of  the  Y's,  the  pivot  is  moved  up  one  side  of  the  sloping  Y,  and 
the  telescope  thereby  changed  in  azimuth.  Now,  if  the  weight  of  the  telescope 
be  counterpoised  by  springs,  the  pivot  is  lifted  still  higher  and  the  resulting 
change  in  azimuth  becomes  greater.  Not  only  that,  but  the  intricacy  of  the 
adjustment  thereby  introduced,  conflicts  very  much  with  the  bearings  of  the 
vertical  axis.  I  saw  no  form  of  reversing,  that  should  be  applied  to  any  first- 
class  theodolite,  or  to  any  theodolite  which  might  be  used  at  a  distance  from 
the  workshop  of  a  skilled  mechanic.  The  same  amount  of  labor  otherwise 
expended  upon  the  same  instrument,  would  largely  increase  its  value  for  honest 
work.  This  form,  as  well  as  the  prism  telescope,  must  however  be  demanded 
by  observers  or  the  makers  would  hardly  continue  their  manufacture. 

Another  fashion  that  seems  to  prevail  is  the  use  of  microscope  micrometers 
upon  small  theodolites  of  eight  inches  diameter,  and  even  in  some  cases  of  only 
six  inches.  These  microscope  micrometers  read  the  circle  to  one  or  to  two 
seconds;  but  in  every  such  case  the  telescopic  power  and  the  transit  axis  level 
were  vastly  inferior  to  such  delicacy,  and  therefore  the  contradiction  existed 
that  the  instrument  maker  expected  the  observer  to  measure  what  he  could  not 
see.  In  fact,  I  think  it  may  be  safely  asserted  as  a  rule  that  the  telescopic  power 
of  all  the  instruments  examined  was  inferior  to  the  other  parts. 

And  still  another  fashion  is  in  the  use  of  the  universal  or  Altazimuth  in- 
strument. The  combination  of  two  or  more  instruments  for  differeut  purposes 
into  one  instrument  for  all  the  required  purposes  is  difficult  and  doubtful  even 
in  machines  of  industry;  but  when  every  new  piece  and  movement  introduces 
a  fresh  source  of  error  into  a  delicate  instrument  where  the  constant-study 
should  be  really  to  decrease  them,  the  combination  must  be   clearly  shown  not 


only  to  possess  freedom  from  additional  errors,  but  to  give  better  results  or 
as  good  results  as  two  different  and  separate  instruments  at  less  cost  of  con- 
struction, less  weight  for  transportation  and  mounting,  and  greater  rapidity  of 
manipulation.  For  certain  classes  of  work,  the  universal  instrument  has,  with- 
out doubt,  some  decided  advantages;  but  for  such  work  as  is  demanded  in  the 
primary  and  main  triangulation  of  the  United  States  they  are  essentially  un- 
suited.  Nor  would  our  surveys  willingly  adopt  the  eccentrically  placed  tele- 
scopes of  the  theodolites  used  in  the  Prussian  Geodetic  Survey,  although  the 
character  of  that  wcrk  stands  very  high;  if  any  series  of  observation  is  broken 
it  cannot  be  utilized  without  extra  calculation;  and  a  loss  of  time  means  a  loss 
of  money.  The  multiplicity  of  parts  in  some  of  the  instruments  on  exhibition 
was  astonishing.  I  was  attracted  by  the  complication  of  an  instrument  having 
a  circle  of  about  eight  inches  diameter,  and  although  I  could  see  little  more 
than  one  side  of  the  instrument  I  counted  no  less  than  93  screws  of  all  kinds, 
whilst  others  seemed  evolved  and  contrived  from  the  inner  consciousness  of 
some  closet  professor. 

In  one  large  combination  instrument  in  the  exhibition,  there  were,  among 
other  curious  features,  two  small  lamps  for  illuminating  the  horizontal  circle 
under  the  three  microscopes;  one  for  the  telescope,  and  one  for  reading  the 
level  and  the  two  microscopes  for  the  vertical  circle,  together  with  a  multipli- 
city of  mirrors  to  reflect  the  lights.  No  matter  howsoever  small  the  heat  ot  the 
flames  might  be,  here  was  the  introduction  of  souces  of  error  that  wou'd  tend 
to  complicate  and  mask  the  other  defects  of  the  instrument. 

I  had  the  opportunity  of  studying  many  of  the  recent  and  varied  forms  of 
portable  transit  instruments.  Some  there  were  that  never  should  have  been 
permitted  to  leave  a  workshop;  others  aiming  at  great  stability  by  the  use  of  very 
heavy  cast-iron  stands,  yet  introducing  an  element  of  error  in  having  their  ad- 
justments for  level  and  azimuth  at  the  base.  This  seems  very  much  like 
erecting  a  great  solid  building  upon  a  movable  foundation.  It  is  granted  that 
in  the  usual  form  of  movable  Y's  tor  the  adjustment  of  the  transit  axis  level 
and  of  azimuth,  two  fertile  sources  of  error  exist,  but  many  years  since  I 
readily  and  successfully  overcame  the  difficulty  by  tightly  clamping  either  mov- 
able Y  alter  the  last  mechanical  correction  has  been  made  to  the  adjustments. 
Troughton  and  Simms  have  in  part  since  used  a  similar  application.  The  frames 
of  the  later  Coast  Survey  transit  instruments  are  emphatically  portable  from 
their  form  and  weight  of  metal ;  their  telescopes  have  generally  greater  light- 
collecting  power  than  the  portable  transits  examined,  whilst  the  character  of 
the  results  is  fully  established  by  the  rigorous  method  of  discussing  them.  By 
the  adoption  of  four  foot-screws  1  have  secured  remarkable  firmness;  whilst  the 
double  frame  gives  no  ,only  great  facilities  for  preliminary  adjustments  in  the 
meridian,  but  enables  the  trans" t  to  be  used  for  a  latitude  instrument  by  the 
Talcot  method.  Some  of  the  portable  transits  in  Europe  hardly  bear  out  that 
character,  and  would  not  be  adopted  in  the  mountains  where  our  geodetic 
work  is  being  carried;  it  was  very  evident  that  cast-iron  was  cheap,  transpor- 
tation easy,  and  time  no  object.  And  I  found  severer  criticisms  than  mine 
passed  upon  particular  instruments  (designed  for  great  surveys)  by  some  whose 
opinions  have  much  weight  in  the  geodetic  world;  whilst  one  well  known 
observer  confessed,  that  were  he  to  design  a  new  instrument  it  would  not  have 
the  form  of  that  which  he  had  planned,  constructed  and  already  used. 

Although  I  made  few  efforts  to  examine  the  manufacture  of  lenses  for  tel- 
escopes and  microscopes,  those  which  I  did  see  were  generally  of  superior 
character.  I  was  very  much  impressed  with  the  thorough  skill  and  knowledge 
of  Shroeder,  of  Hamburg,  who  was  making  the  lenses  for  the  15-inch  equa- 
torial of  the  new  Potsdam  observatory.  An  examination  of  some  of  his  smaller 
instruments  revealed  marvelous  precision  of  figure,  whilst  his  means  of  testing 
the  curvature  of  the  lens  was  beyond  anything  I  had  seen  or  known.  The 
computations  for  the  curvatures  of  the  lenses  are  very  elaborate  and  exhaustive. 

Without  going  into  details  of  telegraphic  longitude  apparatus,  electrical 
clocks  and  chronometers,  etc.,  I  may  mention  that  I  examined  the  base  ap- 
paratus of  Brunner,   being  constructed  for  the  Spanish  Geodetic  Survey;  and 


6 

had  the  greatest  pleasure  in  examining  the  original  base  apparatus  of  Bessel  in 
the  Prussian  geodetic  office.  The  Brunner  apparatus  aims  at  the  simplicity  of 
the  Borda  thermometer,  and  is  therefore  composed  of  two  four-meter  bars  of 
different  metals,  having  largely  different  co-efficients  of  expansion.  The  lower 
bar  is  platinum;  immediately  above  it  is  the  brass  bar  of  the  same  cross  sec- 
tion. At  one  extremity  of  the  compound,  or  rather  composite  bar,  the  two  bars 
are  joined,  but  are  free  towards  the  other  extremity.  At  the  free  ends  the 
upper  bar  has  a  slot  in  it  that  receives  a  piece  soldered  to  the  lower  bar, 
whereby  the  upper  surface  of  this  piece  is  flush  with  the  upper  surface  of  the 
brass  bar.  Upon  one  is  cut  a  series  of  graduated  lines;  upon  the  other  a 
vernier.  "When  the  bars  expand  or  contract  through  increase  or  decrease  of 
temperature  they  do  so  unequally,  and  the  difference  of  that  change  is  read  and 
is  determined  by  the  scale  and  vernier. 

The  base  bars  of  Bessel  are  four  in  number,  and  each  composite  bar  is 
formed  of  two  bars  of  different  metals  of  different  rates  of  expansion  for  equal 
increments  of  heat.  In  this  case  the  lower  bar  is  iron  and  the  upper  is  zinc. 
One  end  of  each  system  is  formed  by  the  junction  of  the  two  bars,  and  thence 
the  bars  are  free.  Instead  of  a  scal-»  upon  one  and  a  vernier  upon  the  other, 
the  zinc  bar  terminates  close  to  a  small  projection  upon  the  upper  surface  of 
the  iron  bar,  and  the  difference  of  expansion  was  measured  by  the  insertion  of 
long  graduated  wedges  of  glass  in  the  space  between  the  end  of  the  zinc  bar  and 
the  iron  stud. 

Either  of  these  forms  of  apparatus  has  great  merit  in  the  extreme  simplicity 
of  the  form  and  fewness  of  parts,  and  it  appears  to  me  that  with  thorough  skill 
in  the  observer,  accuracy  of  comparison  with  the  given  standard  bar,  simplicity 
and  stability  of  the  supporta,  and  the  proper  means  of  effecting  the  contacts, 
the  method  is  susceptible  of  great  accuracy.  But  I  was  very  far  from  being 
satisfied  with  the  character  of  the  supports  and  the  methods  and  appliances  of 
measurement.  These,  however,  I  discuss  fully  in  my  official  report  to  the  Su- 
perintendent of  the  Coast  Survey. 

Without  detaining  the  attention  of  the  Academy  any  longer,  I  may  mention, 
in  brief,  the  general  conclusion  to  which  I  arrived.  While  I  saw  much  of  deep 
interest,  there  was  no  single  instrument  that  1  would  unreservedly  recom- 
mend for  adoption  in  its  entirety.  What  I  principally  learned  was  really  wJiat 
not  to  copy.  And  whilst  awarding  high  credit  to  the  instrument  makers  of 
Europe,  and  keenly  alive  to  their  courtesy,  I  am  convinced  that  we  do  not 
need  to  go  to  Europe  for  geodetic  instruments,  although  we  should  possess 
copies  of  their  finest  efforts  for  comparison  with  our  own.  With  our  own  ob- 
servers and  mechanicians  working  in  harmony  and  striving  to  attain  the  highest 
excellence,  I  firmly  believe  that  we  are  fully  competent  to  lead  in  this  scien- 
tific effort.  Both  parties  fully  appreciate  the  fundamental  requisites  to  success, 
and  the  character  of  the  geodetic  work  of  the  United  States  demands  that  the 
mechanical  means  shall  be  adequate  to  the  delicate  manipulation  and  require- 
ments of  the  most  skillful  observers. 


^ 


